Polyacrylamide Bead Sensors for in vivo Quantification of Cell-Scale Stress in Zebrafish Development

Summary

This preprint from the group of Jochen Guck provides an elegant method to accurately assess and quantify compressional forces in real-time in vivo.

The new methodolody is based on cell-like polyacrylamide (PAAm) beads sensors, which have defined size and elastic properties. Interestingly, the PAAm have been designed to mimic real cells:

– they display a uniform particle size of 17 µm;

– the elastic modulus was chemically adjusted to mimic the typical stiffness of eukaryotic cells (1,8 kPa);

– they have been rendered bioadhesive and fluorescent (thus trackable in vivo) by coating with Poly-L-Lysine conjugated with Cy3 fluorophores.

– they are compressible and volume changes can be used to assess bead elasticity and deformations at small compressions.

Such properties (size, compressibility, elastic modulus, biocompatibility and fluorescence) thus provide a new and elegant method for assessing bead deformations in vivo.

The study also provides a computational method for tracking bead-mediated stress sensing. This method, named COMPAX (for computational analysis-based cell-scale stress sensing) is composed of 3 steps :

1. registration of deformed PAAm beads based on 2D confocal images
2. comparison of the deformed configuration to the undeformed one (the undeformed bead being considered as a sphere with a mean diameter of 17 µm
3. Finite Element simulation to obtain stress distribution over the deformed bead

After numerically validation the COMPAX method, the preprint further details that PAAm beads can sense mechanical stress in aggregates of mesenchymal stem cells in vitro. The authors then injected the PAAm beads in the developing neural plate of a zebrafish embryo. Doing so, they were capable of quantifying the deformations of embedded PLL-Cy3 PAAm beads by computational reconstruction. The method allows now to estimate the direction of main shape changes during neural rod formation in vivo in zebrafish embryos

Bigger picture

This work beautifully demonstrates that functionalized PAAm beads can be efficiently injected into living embryos to quantitatively assess mechanical stress in vivo. Numerical comparison of the bead deformation from the spherical shape of the reference configuration allows to quantitatively probe pressure changes as well as the direction of main shape changes.

Such work expands on preexisting methods for interrogating mechanical stress in vivo. While most of the techniques could only be applied to in vitro situations (such as atomic force microscopy or micropipette aspiration assay) or used FRET-based biosensor methods that are not compatible with cell-scale stress, recent work had demonstrated the power of biocompatible oil microdroplets or elastic round microgels. While oil microdroplets are incompressible, the accuracy of the second method strictly relies on measurements of stress-free microgels with spherical shape.

This new method described within this preprint will very likely provide a useful tool to probe spatiotemporal mechanical stress during developmental processes.

Open questions

This method is perfectly suited for tracking morphogenetic processes in developing embryos, but not only. Indeed, it would be interesting to test such PAAm beads in other physiological and pathological scenarios. In particular, one could probe mechanical stress during tumor growth and invasion using the PAAm beads.

From the experimental point of view, the next steps could aim at:

– testing bead deformations in other developing tissues

– testing bead deformations in other animal small organism models (and organoïds, as proposed by the authors)

– testing the compatibility of such method with intravital imaging (and computation) in mouse models

Tags: beads, biomechanics, development, pressure, stress, zebrafish

doi: https://doi.org/10.1242/prelights.4869

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